Fix GradleConsoleAnnotator when Timestamper is globally enabled

[alextu]

Fixes JENKINS-72411 by detecting the timestamp prefix and shift the Gradle notes.
It should always be a complete timestamp and its format should not change (afaik it's changed by a formatter later on when displaying the console).

Testing done

• Unit test for detecting the timestamp prefix
• Integration test to check the expected Gradle notes. The left menu could not be part of this test as it's loaded separately with an ajax request. I checked it manually though.

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